Hi,
Go to the NYC MTA site, NYC subway website and print out a copy of the stations and route map of their subway system. Carry that with you and you should have no trouble navigating from one line or one division to another. It will also show you where the stations are that have free transfer privileges. Right, you can transfer from one line, or division, to another free of charge.
NYC MTA, the subway system has three divisions; IRT, Interborough Rapid Transit, BMT, Brooklyn Manhattan Transit, and IND, Independent. As a kid my buddies and I prided ourselves on being able to go from one line and one division to another and still return home, for one carfare. You couldn't lose us in those days. Great game, great rides and we often spent the day on the subways.